There are actually two species of koa native to Hawaiʻi. The large forest koa is well known around the world for the beautiful hard wood. Koa's smaller cousin, koaiʻa, that once grew in the lowlands of most of the main Hawaiian Islands, has an even harder wood that is much prized for its gnarled grain. But these trees are now rare and the wood is seldom seen in wood working.

Koaiʻa is usually a better choice in the lowland urban landscape than koa because of its smaller stature, natural lowland dry habitat, drought tolerances, and more pest resistant properties than koa. [4,11]

Trees inoculated with rhizobia tend to be more vigorous. [1]

Additional Fragrance Information

Koaiʻa and koa have a distinctive aroma from the root area from a bacterium called rhizobia present in the soil and roots. An ammonia-like scent is given off during the nitrogen-fixing process. [1]

The pungent aroma is perhaps best appreciated by those who regularly work with this species. Otherwise, it generally has a non-appealing scent. [David Eickhoff, Native Plants Hawaiʻi]

Seedlings can be fertilized 2 or 3 weeks after secondary growth with 8-8-8 or 10-10-10. Since koaiʻa are nitrogen-fixing trees, additional nitrogen is not necessary when phyllodes, or sickle-shaped leaves, begin to appear.

Pruning Information

Trees should not be pruned unless necessary. Lower branches do self-prune. If pruning is required, it should be minimal, done properly and without excessive injury to the tree. [4]

Water Requirements

Dry

Additional Water Information

Koaiʻa can tolerate both dry and moist watering conditions but do best under dry conditions. These small trees have light to moderate water needs. Once established, water once or twice a month in dry season. These xeric trees will not tolerate water-logged soil.

Soil must be well drained

Yes

Light Conditions

Full sun

Partial sun

Additional Lighting Information

Trees do best in full sun.

Spacing Information

Koaiʻa should be planted at least 20 feet apart.

Tolerances

Drought

Wind

Soils

Clay

Cinder

Organic

Limitations

These trees do not do well in very wet soils. Surface roots of koaiʻa will not tolerate high foot traffic, causing damage that will affect the health and vigor of the trees themselves.

The rare koaiʻa naturally grow in open, dry habitats. The Koaiʻa Tree Sanctuary in Kohala (Hawaiʻi Island) is an excellent place to see these trees in one of their last natural habitats. Many grow along the road to Waimea. Sadly, much of koaiʻa habitat has been converted to pasture. [2]

"Lānaʻi has very little koa," notes botanist George Munro. But "koaiʻa can be found widely scattered on the dry country." [7]

Given its natural range throughout the rest of the Main Islands, koaʻa may have possibly been found on Oʻahu in the past as well, though no specimens have been confirmed there.

Koaiʻa belong to the third largest plant family, the Pea or Legume family (Fabaceae). There are two endemic species of Acacia in the Hawaiian Islands: Acacia koaia and A. koa.

Etymology

The generic name Acacia is derived from the Greek, akakia, the name for Acacia arabica, ultimately from akis or ake, a Greek word meaning "a sharp point" and referring to the thorns of this particular plant.

The specific epithet koaia is from the Hawaiian word for this tree.

Hawaiian Names:

Koaiʻa and Koaiʻe are the given names for this species. Regarding Koaiʻe, it can figuratively mean a person or anything from the upland country. [10]

These small trees are nitrogen-fixing and thus very beneficial to a healthy native Hawaiian environment. The growth of koaiʻa is due to the symbiotic relationship with a special bacteria called rhizobia that live in association with the root system. The bacteria convert, or fix, nitrogen from the air into usable nitrogen fertilizer for plants. The leaves, flowers and branches also provide nitrogen for understory plants in the area.

Some of the major physical differences between koa (Acacia koa) and koaiʻa (A. koaia) are:

Koaiʻa are overall smaller and more compact trees with a denser, harder and more gnarled wood than koa.

Koaiʻa usually have straighter and narrower phyllodes (expanded petioles) than koa.

The seeds of koaiʻa are arranged in pods end-to-end (vertical) as opposed to a side-by-side (horizontal) arrangement with koa.

Otto Degener uses ʻakoa as an additional name for koaiʻa. [3]

Early Hawaiian Use

In general, the dense reddish brown wood is harder than koa and was used by early Hawaiians for short spears (ihe), long spears (pololū), fish lures (lāʻau melomelo), shark hooks (makau manō) with bone points, bait sticks in fishing, fancy paddles (hoe), house (hale) construction, ʻukēkē (musical bow), calabashes (ʻumeke), [5] and the ʻiʻe kūkū --the final beater to smooth out the kapa. [5,8]

Koaiʻa was not used for making canoes (waʻa) because it produced curly grained wood. [9]

Lei:

One older source (Charles Gaudichaud,1819) states that Hawaiians "used all fragrant plants, all flowers and even colored fruits" for lei making. The red or yellow were indicative of divine and chiefly rank; the purple flowers and fruit, or with fragrance, were associated with divinity. Because of their long-standing place in oral tradition, the leaves and flowers of koaiʻa were likely used for lei making by early Hawaiians, even though there are no written sources. [6]

Medicinal:

The crushed koaiʻa leaves were mixed with other plant materials and used in a steam bath for skin disorders.

Modern Use

Koaiʻa is sometimes seen in home and commecial landscapes. Hopefully, it will become a commonly used urban tree since it does so well in lower elevations.

Though the wood was apparently readily available to the early Hawaiians, only later in time was it used to make furniture. [10]

The phyllodes (sickle-shaped "leaves") are still used to make a nice lei. [6]